Partial discharge characterization of streamers in liquid nitrogen under applied AC voltages

Streamer growth has been examined with the aim to further understand the issues related to using liquid nitrogen as a part of a dielectric system within power apparatus using high temperature superconducting conductors (HTSC). An experiment has been designed to image streamer events in liquid nitrogen for applied electric fields with alternating current using a technique that allows time correlated partial discharge signal and image capture of the density-change streamers. This method employs high speed digital imaging at 5,000 and 30,000 frames per second with stroboscopic backlighting of samples. Samples are point-plane with divergent electric field geometry and consists of liquid nitrogen and a composite solid barrier preventing total field collapse during discharge. Magnitude of apparent charge and phase relationship of partial electrical discharge in liquid nitrogen have been correlated to simultaneously captured images of the resulting density-change streamer. Images of streamers at different points on the applied wave are shown to highlight that different physical mechanisms of propagation occur, phi-q-n plots of partial discharge activity are presented for a range of applied voltages, bulk liquid temperatures and pressures. Analysis of obtained results support the hypothesis that positive filamentary streamer growth is caused by ionization processes occurring in the liquid phase.